Everything is by ETF. And I submit my Federal return electronically. So from both of your responses, there really isn't an explanation yet. The solution to this of course (I hope) is to get my estimated taxes paid in full every quarter. It's also good to know that it's not uncommon to get a check or bill back for a few bucks. I know for my state taxes, I routinely get a check back of have to make a payment for a couple dollars most years. That is understandable. This is the first year I've owed anything related to underpayment of the penalty. I'm pretty sure that all my dates on the payments submitted on my tax forms are correct, and all of that is done electronically and by ETF. And my federal estimated payments, which is ALL of my federal payments, are done via Direct Pay.
